Re: [RFC PATCH] arm64: defconfig: Disable fine-grained task level IRQ time accounting

From: Valentin Schneider
Date: Mon Aug 03 2020 - 11:47:12 EST

On 03/08/20 16:13, Thomas Gleixner wrote:
> Vladimir Oltean <olteanv@xxxxxxxxx> writes:
>>> 1) When irq accounting is disabled, RT throttling kicks in as
>>> expected.
>>> 2) With irq accounting the RT throttler does not kick in and the RCU
>>> stall/lockups happen.
>> What is this telling us?
> It seems that the fine grained irq time accounting affects the runtime
> accounting in some way which I haven't figured out yet.

With IRQ_TIME_ACCOUNTING, rq_clock_task() will always be incremented by a
lesser-or-equal value than when not having the option; you start with the
same delta_exec but slice some for the IRQ accounting, and leave the rest
for the rq_clock_task() (+paravirt).

IIUC this means that if you spend e.g. 10% of the time in IRQ and 90% of
the time running the stress-ng RT tasks, despite having RT tasks hogging
the entirety of the "available time" it is still only 90% runtime, which is
below the 95% default and the throttling doesn't happen.

I don't know if considering IRQ time in some way or another in
sched_rt_runtime_exceeded() really is a way out here.

> Thanks,
> tglx